Single-carrier (SC)-frequency division multiple access (FDMA), which is similar to OFDM, but is capable of reducing power consumption and power amplifier cost of a portable terminal by decreasing a peak to average power ratio (PAPR) has been adopted in an uplink of LTE standardized as part of a next-generation mobile communication standard in 3GPP.
SC-FDMA is a technique that is very similar to OFDM in which a signal is divided and transmitted in sub-carriers using a fast Fourier transformer (FFT) and an inverse-FFT (IFFT) for transmission. Further, use of a guard interval (cyclic prefix) enables a simple equalizer to be used in a frequency domain with respect to inter-symbol interference (ISI) caused by multi-path fading, as in an existing OFDM technique. However, power efficiency of a transmitter has been enhanced by reducing a PAPR at a transmitter stage by about 2 to 3 dB using an additional unique technique.
A problem associated with an existing OFDM transmitter is that frequency-axis signals loaded on respective sub-carriers are transformed into time-axis signals by the IFFT. Since the IFFT takes a form in which the same operations are in parallel, PAPR increase is caused.
FIG. 1 is a block diagram for explaining an SC-FDMA transmission scheme that is an uplink access scheme adopted in 3GPP LTE.
In order to solve such a problem, in SC-FDMA, a signal is first processed by a discrete Fourier transformer (DFT) 102 before the signal is mapped to a sub-carrier, as shown in FIG. 1. A signal spread (i.e., pre-coded) by the DFT is mapped 103 to the sub-carrier, and then transformed into a time-axis signal by the IFFT 104. In SC-FDMA, a PAPR of the time domain signal from the IFFT 104 is not greatly increased due to correlations between the DFT 102, the sub-carrier mapping 103 and the IFFT 104, unlike OFDM. Accordingly, SC-FDMA is advantageous in transmission power efficiency.
That is, SC-FDMA has robustness for a multi-path channel by having a similar structure to OFDM, and enables efficient use of a power amplifier (PA) by fundamentally resolving a problem of the existing OFDM that an IFFT operation increases the PAPR. Meanwhile, SC-FDMA is also called a DFT-spread-OFDM (DFT-s-OFDM)
Further, standardization of LTE-Advanced, which is an enhanced version of LTE, has been actively conducted by the 3GPP group, and an SC-FDMA technique and an OFDM technique have been competing in the LTE-Advanced standardization process, as in the LTE standardization process. However, a clustered DFT-s-OFDM scheme allowing for discontinuous resource allocation has been adopted.
FIG. 2 is a block diagram for explaining a clustered DFT-s-OFDM transmission scheme adopted as an uplink access scheme in an LTE-Advanced standard.
An important characteristic of the clustered DFT-s-OFDM scheme is that the clustered DFT-s-OFDM scheme can flexibly cope with a frequency selective fading environment by enabling frequency selective resource allocation.
Meanwhile, in the case of an LTE uplink, a demodulation reference signal (DM-RS) used for channel estimation for demodulating a physical uplink control channel (PUCCH) and a physical uplink shared channel (PUSCH) is generated and transmitted using a constant amplitude zero autocorrelation (CAZAC) sequence. In order to identify the DM-RSs between terminals for multi-user MIMO (MU-MIMO), DM-RS orthogonality between the terminals is realized by using, in the DM-RS, the CAZAC sequence having a different cyclic shift value between the terminals.
In this case, since the clustered DFT-s-OFDM scheme adopted as an uplink access scheme of the LTE-Advanced allows discontinuous resource allocation, unlike SC-FDMA that is an uplink access scheme of conventional LTE, transmitted uplink data may be divided into several clusters.
Accordingly, the clustered DFT-s-OFDM scheme adopted as the uplink access scheme of the LTE-advanced requires a method of generating and transmitting a DM-RS in a cluster unit, unlike SC-FDMA that is the uplink access scheme of the conventional LTE.